1. Field of the Invention
The present invention generally relates to an actuator and, more particularly, to an electrostatic actuator of an inkjet head provided in an inkjet recording apparatus.
2. Description of the Related Art
An inkjet recording apparatus is used as an image recording apparatus in a printer, a facsimile machine or a copy machine or other such apparatus. The inkjet head includes a plurality of nozzles each of which discharges droplets of ink, a plurality of discharge chambers each of which is connected to a respective one of the nozzles and a plurality of actuators each of which pressurizes ink stored in a respective one of the discharge chambers. The discharge chamber may be a pressure chamber, a pressurizing chamber, a fluid chamber or an ink chamber. Droplets of ink are discharged from each of the nozzles by driving each of the actuators which pressurize the ink stored in the discharge chamber.
An ink-on-demand method is often used in such an inkjet head. According to the ink-on-demand method, droplets of ink (recording fluid) are discharged only when a recording operation is performed, that is, the droplets of ink are actually ejected onto a recording sheet. The inkjet head is classified into one of a plurality of types with respect to generation of droplets of ink and a method of controlling a direction of movement of the droplets of ink.
Japanese Patent Publication No. 4-52214 and Japanese Laid-Open Patent Application No. 3-293141 disclose an electrostatic inkjet head which uses an electrostatic actuator. In this type of inkjet head, a fluid chamber and a diaphragm are formed by etching a first silicon substrate (diaphragm substrate) so that the diaphragm defines a wall of the fluid chamber. A second substrate (electrode substrate) having an electrode is located under the diaphragm substrate so as to define an electrostatic actuator. The diaphragm is deformed by an electrostatic force which is generated by a voltage applied between the diaphragm and the electrode. As a result, the volume of the fluid chamber is changed which results in droplets of ink which are stored in the fluid chamber being ejected.
The driving method of such an electrostatic actuator is classified into one of the two types in accordance with a range of displacement of the diaphragm when the diaphragm is deformed by an electrostatic force. One is a method, such as that disclosed in Japanese Laid-Open Patent Application No. 7-214770, in which the diaphragm is displaced by an amount corresponding to the extent that the diaphragm contacts the electrode. This method is referred to as a contact drive method. The other is a method in which the displacement of the diaphragm is limited to the extent that the diaphragm does not contact the electrode. This method is referred to as a non-contact drive method.
The displacement of the diaphragm driven by the contact drive method is much greater than that of the diaphragm driven by the non-contact drive method in a case in which a gap between the diaphragm and the electrode is the same. Accordingly, the contact drive method is advantageous in that the diaphragm can be driven by a lower voltage than that used in the non-contact drive method when discharging the same amount of ink. Additionally, the contact drive method is superior to the non-contact drive method in that the amount of ink discharged by the displacement of the diaphragm at one time can be uniform since the displacement of the diaphragm is limited by the distance (gap size) between the diaphragm and electrode.
However, according to the contact drive method, the displacement of the diaphragm is larger than that of the non-contact drive method when the same amount of ink is discharged. As a result of such a large displacement of the diaphragm, the contact drive method has a problem in that damage and destruction easily occurs in the diaphragm, especially, due to fatigue. Additionally, the amount of ink discharged from the inkjet head varies greatly due to fluctuation in the voltage applied between the diaphragm and the electrode.
In order to overcome the problems described above, preferred embodiments of the present invention provide an electrostatic actuator for use in an inkjet head of an inkjet recording apparatus, wherein the actuator is constructed such that a diaphragm is prevented from being damaged or destroyed due to an excessive stress or fatigue, while also maintaining a constant amount of displacement of the diaphragm even when a voltage fluctuation occurs.
According to one preferred embodiment of the present invention, an electrostatic actuator includes a diaphragm which is elastically deformable by an electrostatic force and an electrode facing the diaphragm with a predetermined air gap defined therebetween so as to generate the electrostatic force. The electrode has a width that is smaller than a width of the diaphragm at all points along a longitudinal direction of the electrode. The electrode is located under the diaphragm and arranged such that when the electrode is vertically projected onto the diaphragm, the projected electrode is situated within the diaphragm in a direction of the width of the electrode.
In the electrostatic actuator according to preferred embodiments of the present invention, when the electrode is vertically projected onto the diaphragm, the entirety of the projected electrode may be situated within the diaphragm.
Additionally, a length d of the air gap at an arbitrary point p1 along a side of a top surface of the electrode and a distance L between the point p1 and a point p2 along a side of the diaphragm, wherein point p2 is closest to the point p1, may preferably satisfy a relationship L/dxe2x89xa710. More preferably, the actuator is constructed to satisfy a relationship L/dxe2x89xa750.
Additional elements, features and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention when read in conjunction with the accompanying drawings.